feat(subagent): sibling delegations are concurrency-safe

One assistant message carrying several subagent calls now overlaps them
under the rolling pool (maxParallelToolCalls) instead of serializing each
foreground delegation behind an exclusive barrier; results still commit
in model order.

tool-subagent declares isConcurrencySafe: () => true for every call form:
children work in their own sessions, a run never mutates the parent
session, and sibling workspace coordination already belongs to the model
for background, continuable, and workflow children. The former
serial-execution pin flips to a parallel pin plus a gated genuine-overlap
test, and the authored subagent-parallel snapshot pins the assembled
transcript (tool/call, tool/call, tool/result, tool/result) with
interchangeable twin children, which replay race-free under the existing
first-call binding and createdAt harvest ordering.

The provider seam now requires concurrent starts and continuable
preparations for distinct children to isolate operation-local state,
cancellation, settlement, and cleanup; a continuable gate test proves a
cancelled preparation leaves no Agent or durable Session while its
sibling persists independently.

Closes deepseek-harness/deepseek-harness#1489

packages/subagent/tool-subagent/README.zh.md

@@ -29,7 +29,7 @@
 
 ## 并发
 
-前台调用和后台调用均互斥。子 agent 可能共享父级工作区或外部资源，一元分类器无法证明同级委派的效果彼此不相交。见 [并行工具调用 Agent Note](../../../.agents/notes/implemented/feature/2026-07-10-parallel-tool-call-execution.md)。
+前台调用和后台调用均并发安全：同一条 assistant 消息中的同级委派会在循环的滚动池（`maxParallelToolCalls`）下重叠执行，结果仍按模型顺序提交。子 agent 在各自的会话中工作，一次运行绝不变更父会话；一次性后台形态对父级拥有状态的唯一写入是注册一个 Task——这是一次同步、可交换、能容忍并发分发的插入，因此重叠的后台调用按分发竞态顺序获得各自的 task id。协调同级工作区效果由模型负责，正如模型已经对后台和可继续子 agent 所承担的那样。见 [并行 subagent Agent Note](../../../.agents/notes/implemented/feature/2026-08-09-parallel-subagent-delegations.md) 和 [并行工具调用 Agent Note](../../../.agents/notes/implemented/feature/2026-07-10-parallel-tool-call-execution.md)。
 
 ## 模型体验